Determination of the cross section for (n,p) and (n,α) reactions on 165Ho at 13.5 and 14.8 MeV

Applied Radiation and Isotopes 98 (2015) 40–43

Contents lists available at ScienceDirect

Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso

Determination of the cross section for (n,p) and (n,α) reactions on 165Ho at 13.5 and 14.8 MeV Junhua Luo a,n, Li An b, Li Jiang b, Long He a a b

School of Physics and Electromechanical Engineering, Hexi University, Zhangye 734000, China Institute of Nuclear Physics and Chemistry, Chinese Academy of Engineering Physics, Mianyang 621900, China

H I G H L I G H T S


27Al(n,α)24Na was used as a monitor for neutron fleunce.
The cross sections for the 165Ho(n,p)165Dy and 165Ho(n,α)162Tb reactions were measured at 13.5 and 14.8 MeV neutron energies.
Nuclear reaction codes TALYS-1.6 and EMPIRE-3.2 Malta were used to model the reactions.
Inconsistency with previous data and with model calculations are noted.

art ic l e i nf o

a b s t r a c t

Article history: Received 22 August 2014 Received in revised form 11 January 2015 Accepted 17 January 2015 Available online 21 January 2015

Activation cross-sections for the 165Ho(n,p)165Dy and 165Ho(n,α)162Tb reactions were measured by means of the activation method at 13.5 and 14.8 MeV, to resolve inconsistencies in existing data. A neutron beam produced via the 3H(d,n)4He reaction was used. Statistical model calculations were performed using the nuclear-reaction codes EMPIRE-3.2 Malta and TALYS-1.6 with default parameters, at neutron energies varying from the reaction threshold to 20 MeV. Results are also discussed and compared with some corresponding values found in the literature. The calculational results on the 165Ho(n,α)162Tb reaction agreed fairly well with experimental data, but there were large discrepancies in the results for the 165 Ho(n,p)165Dy reaction. & 2015 Elsevier Ltd. All rights reserved.

KeyWords: Holmium-165 Neutron cross sections Excitation function EMPIRE-3.2 Malta TALYS-1.6

1. Introduction Holmium can absorb nuclear fission neutrons, and is used as a burnable poison to regulate nuclear reactors (Emsley, 2001). However, the cross sections for the 165Ho(n,p)165Dy and 165 Ho(n,α)162Tb reactions have not been so far reported in major nuclear data libraries, such as ENDF/B-VII.1 (ENDF, 2011), JEFF-3.2 (ENDF, 2014), and JENDL-4.0 (ENDF, 2010). The reaction cross sections for 165Ho(n,p)165Dy at around 14 MeV was only previously reported by two workers (Fukuzawa, 1961; Ryves et al., 1990), with large discrepancies in their data. The result of Fukuzawa (1961), using a tritiated zirconium target, a plastic scintillator and associated alpha particle counting, was about 10 times higher than that of Ryves et al. (1990), who utilized a T-Ti target, a Ge–In detector and 56Fe(n,p)56Mn monitor reaction. Therefore, it is necessary to n

Corresponding author. Fax: þ86 936 8283290. E-mail address: [email protected] (J. Luo).

http://dx.doi.org/10.1016/j.apradiso.2015.01.018 0969-8043/& 2015 Elsevier Ltd. All rights reserved.

make further measurements to resolve some of the discrepancies for the cross sections of the 165Ho(n,p)165Dy and 165Ho(n,α)162Tb reactions. In this work, the cross-sections for the 165Ho(n,p)165Dy and 165 Ho(n,α)162Tb reactions were measured at neutron energies of 13.5 and 14.8 MeV, and a gamma-ray counting technique was applied using a high-resolution gamma-ray spectrometer. Measurements were corrected for gamma-ray attenuation, random coincidence, deadtime and fluctuation of neutron flux. The neutron energies in this measurement were determined by the method of Luo et al. (2013). The cross sections were also estimated with the nuclear-reaction codes EMPIRE-3.2 Malta and TALYS-1.6, and compared with experimental data found in the literature.

2. Experiments About 6 g of Ho2O3 (99.99% pure) powder was pressed at 980 MPa, and thin 20 mm diameter samples were obtained. The

J. Luo et al. / Applied Radiation and Isotopes 98 (2015) 40–43

Sample no.2

41

135° Enlarge

0°

d+

Sample no.1

T-Mo target

Al foil Ho2O3 sample

135°accompanying alpha particle tube Fig. 1. Sketch of experimental geometry.

samples were irradiated in contact with the target, sandwiched between standard Al foils (99.999% pure, 0.04 mm thick) of the same diameter utilized to monitor the neutron fluence via the 27 Al(n,α)24Na reaction. Irradiation of the samples was carried out at the K-400 Neutron Generator at the Chinese Academy of Engineering Physics (CAEP), and lasted about 60 min with a yield  3–4  1010 n/(4π s). The sample positions in the experiment are shown in Fig. 1. The groups of samples were placed at 0° or 135° angles relative to the beam direction and were centered about the tritium–molybdenum (T– Mo) target at distances of  40 mm. A T–Mo target was used in the generator, with a thickness of 5.66  10  3 mg/mm2, and the diameter of the active zone was 12 mm. Neutrons were produced by the T(d,n)4He reaction with an effective deuteron beam energy of 136 keV and beam current of 280 μA. During irradiation, the neutron flux was monitored by the accompanying α-particles, so that corrections could be performed for small variations in the yield. An Au–Si surface barrier detector was located at 135° to accompany the α-particle tube palced at a distance of 1.10 m from the target. The cross sections for the 27Al(n,α)24Na monitor reaction were obtained by interpolating the evaluated values reported by Wagner et al. (1990). The neutron energies in this experiment were determined by the cross section ratios for the 90 Zr(n,2n)89m þ gZr and 93Nb(n,2n)92mNb reactions, because the excitation function of 90Zr(n,2n)89m þ gZr has a sharp slope in the 13–18 MeV, whereas that of 93Nb(n,2n)92mNb is almost flat (Luo et al., 2013). The gamma ray activities of 165Dy, 162Tb and 24Na were determined by a high-purity germanium (HPGe) detector (ORTEC, model GEM 60P, crystal diameter: 70.1 mm, crystal length: 72.3 mm) with a relative efficiency of  68% and an energy resolution of 1.69 keV at 1.332 MeV for 60Co. Fig. 2 shows typical spectra acquired from the Ho2O3 samples during the measurement, where the γ-rays of interest are marked. The γ-ray intensities and half-lives used in the analysis are summarized in Table 1 (ENSDF, 2014). The efficiency of the detector was pre-calibrated using various standard gamma sources; namely Co-57, Co60, Cd-109, Ba-133, Cs-137, Eu-152, Am-241 and Ra-226. An absolute efficiency calibration curve was obtained at 95 mm from the surface of the germanium crystal. At this distance the coincidence losses was considered to be relatively small. However, we needed to calibrate the efficiency at 28 mm, the actual counting position used, because of the weak activity of the sample. Therefore, we selected a set of single γ-line sources and placed them at two

Fig. 2. (a) The γ-ray spectrum of holmium about 38 min after the end of irradiation; (b) the γ-ray spectrum of holmium about 12 min after the end of irradiation; (c) background spectrum. Table 1 Neutron induced nuclear reactions on holmium and decay data of associated activation products (taken from ENSDF (2014)). Reaction

Half-life of product

165

Ho(n,p)165Dy 2.3341 h Ho(n,α)162Tb 7.6015 m 27 Al(n,α)24Na 14.9593 h 165

E-threshold (MeV)

Mode of decay (%)

Eγ (keV) Iγ (%)

0.507 0.000 3.249

β  (100) β  (100) β  (100)

361.68 260.07 1368.6

0.84(9) 80(5) 100

positions (95 and 28 mm) successively to measure their efficiency ratios, so that we were able to evaluate the efficiency ratio curve as a function of energy. The absolute efficiency calibration curve at 28 mm was obtained from the calibration curve at 95 mm and the efficiency ratio curve. The measured cross sections were calculated by the following formula given in (Luo et al., 2014):

σx =

[SεIγ ηKMD]0 [λAFC]x σ0 [SεIγ ηKMD]x [λAFC]0

(1)

where the subscript 0 represents the terms corresponding to the monitor reaction and subscript x corresponds to those of the measured reaction; ε is the full-energy peak efficiency of the measured characteristic gamma-ray; Iγ the gamma-ray intensity; η the abundance of the target nuclide; M the mass of sample; D = e−λt1 − e−λt2 the counting collection factor; S = 1 − e−λT the

42

J. Luo et al. / Applied Radiation and Isotopes 98 (2015) 40–43

3. Nuclear model calculations

Table 2 Correction factors for self-absorption in the sample. Gamma- μ/ρ (cm2/g) energy O Ho (keV)

Ho2O3

260.07

0.1137

361.68

0.1000 0.2016

μ(Ho2O3) Samples (cm  1) No. Thickness h (cm)

0.4052 0.3682 1.578 0.1887

3.078

Correction factors

1 2

0.1450 0.1531

1.24 1.25

1 2

0.1450 0.1531

1.19 1.26

growth factor of the product nuclide, T the total irradiation time; t1,t2 the time intervals from the end of the irradiation to the start and end of counting, respectively; A the atomic weight; C the measured full energy peak area; λ the decay constant and F is total correction factor of the activity:

F = fs × f g

(2)

where fs and fg are correction factors for the self-absorption of the sample at a given gamma-energy and in the counting geometry, respectively. The gamma ray attenuation correction factors fs in the Ho2O3 foil and the geometry correction fg were calculated by the following equations, respectively.

fs =

μh 1 − exp ( − μh)

(3)

fg =

(D + h/2)2 D2

(4)

Here μ is the linear attenuation coefficient in Ho2O3 for gamma rays at each of the photon energies E (see Table 2), h is the thickness of the sample and D is the distance from the measured sample to the surface of the germanium crystal. Values for the mass attenuation coefficient, μ/ρ, for the Oxygen and Holmium, which are 0.1137, 0.1000 and 0.4052, 0.2016 cm2/g at 260.07 and 361.68 keV gamma-ray energies respectively (Hubbell and Seltzer, 2013). In the correction factors for the self-absorption of the sample at a given gamma-energy, h in Eq. (3) was taken as the thickness of the sample. The correction factors at 260.07 and 361.68 keV gamma-rays are given in Table 2. The main error sources in our work resulted from counting statistics (13–18%), standard detector efficiency (2–4%), cross sections uncertainties (1.5%), weight of samples (0.1%), self-absorption of gamma-ray (1%) and the cooling and measuring times (0.1–1%), etc. Some other errors originated from the parameters of the measured nuclei and standard nuclei, such as, uncertainties in the branching ratio of the characteristic gamma rays, uncertainties in the half-life of the radioactive product nuclei, and so on. Error analysis was carried out using the quadrature method (Taylor, 1982).

The measured cross sections were compared with theoretical cross sections obtained from two state-of-the-art nuclear reaction codes: EMPIRE-3.2 Malta (Herman et al., 2013) and TALYS-1.6 (Koning et al., 2013). The calculations performed with the nuclear-reaction code EMPIRE-3.2 Malta (Herman et al., 2013) included contributions from direct (DI) reactions, pre-equilibrium (PE), and compound nucleus (CN) reactions. Direct reactions to the low-lying collective states of the deformed nuclei were described by coupled-channels calculations, using an appropriate optical potential (OP) (the ECIS code is used for that purpose). Pre-equilibrium neutron emission was obtained from the multistep compound (MSC) and multistep direct (MSD) theories of Feshbach et al. (1980), while proton PE was treated by the exciton model code DEGAS (Herman et al., 2013). Pre-equilibrium γ-ray emission was obtained from the exciton model code PCROSS (Herman et al., 2013). Particle andγ-ray CN emission were described within the statistical theory of Hauser and Feshbach (1952) using suitable optical model potentials (OP), nuclear level densities (NLD), and γ-ray strength functions (γSF) available in the RIPL-2 database (Http). The calculations were done using the default parameter values. An additional calculation was performed using the nuclear-reaction code TALYS-1.6 (Koning et al., 2013). The optical model parameters for neutrons and protons were obtained by a local potential proposed by Koning and Delaroche (2003). Similarly, the compound nucleus contribution was calculated by the Hauser– Feshbach model (Hauser and Feshbach, 1952). The folding approach of Watanabe (1958) was used for α particles. The twocomponent exciton model developed by Kalbach (1986) was used for calculating the pre-equilibrium contribution. The default values were used for parameters concerning nuclear masses, ground-state deformations, discrete levels, decay schemes, and strength functions.

4. Results and discussion The cross sections measured in this work are given in Table 3. The total uncertainty amounts to between 15% and 45%. For the 165 Ho(n,p)165Dy and 165Ho(n,α)162Tb reactions, in the neutron energy range of 13–15 MeV, the cross section increases with the increasing neutron energy. For the 165Ho(n,p)165Dy reaction, there are only two earlier measurements obtained by Fukuzawa (1961) and Ryves et al. (1990) at 14.2 MeV. At neutron energy 13.5 and 14.8 MeV, the cross sections are reported here for the first time, to the best of our knowledge. The cross section data for the 165Ho(n,p)165Dy reaction are shown in Fig. 3. Regarding the nuclear model calculations, in Fig. 3, the results of EMPIRE-3.2 Malta and TALYS-1.6 are given as continuous lines. From Fig. 3, it can be seen that our results are in agreement with the values for the nuclear model calculation obtained using the code TALYS-1.6, but our values and results of TALYS-1.6 are higher than results of EMPIRE-3.2 Malta. For this

Table 3 Cross-sections measured at 13.5 and 14.8 MeV energies. Reaction

165

Ho(n,p)165Dy Ho(n,α)162Tb 27 Al(n,α)24Naa 165

a 27

Al(n,α)24Na is monitor reaction.

Cross sections (in mb) at various neutron energies (in MeV) 13.5 7 0.2

14.8 70.2

5.8 7 1.0 0.9 7 0.4 125.7 7 0.8 Wagner et al. (1990)

7.5 7 1.1 1.4 7 0.6 111.9 7 0.5 Wagner et al. (1990)

J. Luo et al. / Applied Radiation and Isotopes 98 (2015) 40–43

43

measured data are attributed to measuring methods and experimental conditions.

Acknowledgments We would like to thank the Intense Neutron Generator group at Chinese Academy of Engineering Physics for performing the irradiations. This work was supported by the National Natural Science Foundation of China (Grant no. 11165007) and by the Key Project of Chinese Ministry of Education (No. 211184).

Fig. 3. Excitation function of the

Fig. 4. Excitation function of the

165

Ho(n,p)165Dy reaction.

165

Ho(n,α)162Tb reaction.

reaction, the cross section was previously measured by Fukuzawa (1961) as (41 710) mb at 14.1 MeV, which is a factor 10 greater than our result. However, the technique employed by Fukuzawa, who irradiated a stack of 20 composite Ho2O3 powder foils interleaved with thin plastic scintillators, was considered by Ryves et al. (1990) to be highly suspect. Because the β-activity is difficult to measure accurately with 20 foils piled up alternately with thin plastic scintillators. The 260.07 keV gamma-ray emitted in the 162Tb decay was used to deduce the value of the 165Ho(n,α)162Tb reaction cross section. The measured cross sections for 165Ho(n,α)162Tb reaction are shown in Fig. 4, together with the results of previous experiments (Sakane et al., 1996; Fang et al., 2008; Ryves et al., 1990; Qaim, 1984; Hirose et al., 1996; Prasad et al., 1969). In the energy region between 13 and 15 MeV, agreement between different measured data is very good within their experimental uncertainty. Between 13 and 15 MeV, the TALYS-1.6 and EMPIRE-3.2 Malta calculations agree very well with present data within the data uncertainties.

5. Conclusions We have measured the activation cross-sections for Ho(n,p)165Dy and 165Ho(n,α)162Tb reactions induced by 13.5 and 14.8 MeV neutrons using the latest decay data. The present results were compared with those measured previously and with results of TALYS-1.6 and EMPIRE-3.2 Malta nuclear model calculations with default parameters. Discrepancies with previously reported 165

References ENDF (Evaluated Nuclear Data File), 2010. JENDL-4.0 (Japan): 〈http://www.nndc.bnl. gov/ exfor/endf00.jsp〉. ENDF (Evaluated Nuclear Data File), 2011. ENDF/B-VII.1 (USA): 〈http://www.nndc. bnl. gov/exfor/endf00.jsp〉. ENDF (Evaluated Nuclear Data File), 2014. JEFF-3.2 (Europe): http://www.nndc.bnl. gov/ exfor/endf00.jsp. ENSDF (Evaluated Nuclear Structure Data File), 2014. 〈http://www.nndc.bnl.gov/ ensdf/〉. Fang, K., Xu, S., Lan, C., Xu, X., Kong, X., Liu, R., Jiang, J., 2008. Cross-section measurement for the reactions producing short-lived nuclei induced by neutrons around 14 MeV. Appl. Radiat. Isot. 66, 1104. Feshbach, H., Kerman, A., Koonin, S., 1980. The statistical theory of multi-step compound and direct reactions. Ann. Phys. 125, 429–476. Fukuzawa, F., 1961. Isomeric cross section ratios for 14 MeV neutron reactions. J. Phys. Soc. Jpn. 16, 2371. Hauser, W., Feshbach, H., 1952. The inelastic scattering of neutrons. Phys. Rev. 87, 366. Herman, M., Oblozinsky, P., Capote, R., Trkov, A., Zerkin, V., Sin, M., Carlson, B., 2013. 〈http://www.nndc.bnl.gov/empire/main.html〉. Hirose, T., Tsurita, Y., Iida, T., Takahashi, A., Kasugai, Y., Yamamoto, H., Ikeda, Y., Kawade, K., 1996. Measurement of beta-decay half-lives of short-lived nuclei, Japanese Report to the I.N.D.C.; No.178/U. 202. Hubbell J.H., and Seltzer, S.M., 2013. 〈http://physics.nist.gov/PhysRefData/Xray MassCoef/Table3.html〉. Emsley, John, 2001. Nature's Building Blocks: An A–Z Guide to the Elements. Oxford University Press, USA, ISBN: 0-19-850341-5, pp. 181–182. Kalbach, C., 1986. Two-component exciton model: basic formalism away from shell closures. Phys. Rev. C 33, 818. Koning, J., Delaroche, J.P., 2003. Local and global nucleon optical models from 1 keV to 200 MeV. Nucl. Phys. A 713, 231. Koning, A., Hilaire, S., Goriely, S., 2013. TALYS-1.6, A nuclear reaction program, NL1755 ZG Petten, The Netherlands, 〈http://www.talys.eu〉. Luo, J., An, L., Jiang, L., Liu, Z., 2014. Excitation functions of neutrons induced reactions on terbium from threshold to 20 MeV. J. Radioanal. Nucl. Chem. 300 (2), 819–823. Luo, J., Du, L., Zhao, J., 2013. A method to determine fast neutron energies in large sample. Nucl. Instrum. Methods B 298, 61. Prasad, P.R., Rao, J.R., Kondaiah, E., 1969. Cross sections for (n,2n), (n,α) and (n,p) reactions in rare earth isotopes at 14.2 MeV. Nucl. Phys. A 125, 57. Qaim, S.M., 1984. Activation cross sections of (n,α) reactions at 14.7 MeV in the region of rare earths. Radiochim. Acta 35, 9. Ryves, T.B., Kolkowski, P., Hooley, A.C., 1990. Ho, Pb and Bi cross sections for 14.3 MeV Neutrons. Ann. Nucl. Energy 17, 107. Sakane, H., Matsumoto, T., Iida, T., Takahashi, A., Yamamoto, H., Kawade, K., 1996. Measurement of cross sections producing short-lived nuclei by 14 MeV neutrons – Br, Te, Dy, Ho, Yb. JAERI Reports; No.97,005, 263. Taylor, J.R., 1982. An Introduction to Error Analysis. University Science Books, Mill Valley, CA. Wagner, M., Vonach, H., Pavlik, A., Strohmaier, B., Tagesen, S., Martinez-Rico, J., 1990. Physik Daten-Physics Data, Evaluation of Cross Sections for 14 important Neutron-dosimetry Reactions, Fachinformationszentrum Karlsruhe, Gesellschaft für wissenschaftlich-technische Information mbH, in the Federal Republic of Germany, (No.13-5), Karlsruhe. Watanabe, S., 1958. High energy scattering of deuterons by complex nuclei. Nucl. Phys. 8, 484.